Welcome to the forum.
Firstly, what's your budget? And what level of quality are you looking for?
The trouble with 'do-it-all' lenses is that the optical quality will not be as good as a more specific lens. Also, they usually have rather small apertures at the long end of the zoom. This may not be a problem if you don't shoot sports and don't shoot in low light...but it can be a hindrance at times.
What lens or lenses do you currently have? Are you looking to replace or compliment your current lenses?
There are several 70-300 lenses avaliable. Canon makes a a few cheap ones, as well as one with IS (image stabilization)...which is more expensive but a better lens and having IS, is great.
Sigma & Tamron also make a similar lens.
If you don't need/want 300mm....then there is the 70-200mm group of lenses...which are some of the best zoom lenses avaliable. There is the 70-200 F4L, F4 L IS, F2.8 L & F2.8 L IS. (L is Canon's designation for top quality, Pro, lenses.) These vary in price but they are each much better than the 70-300 lenses.